| A modern communication technology concept,vehicular ad hoc networks(VANETs),have become the hope for providing safer and well-organized transportation in the near future.Because VANETs have been regarded as one of the major technologies to support most future Intelligent Transportation System(ITS).On the other hand,ITS is the major application of VANETs,such as cooperative traffic monitoring,control of traffic flows,blind crossing,prevention of collisions,nearby information services,and r eal-time detour routes computation which are in charge of making safer and more efficient the use of transport networks.Another important application for VANETs is providing internet connectivity,each vehicle is capable of communicating with nearby vehicles and accessing information,so the users can download music,send emails,or play back-seat passenger games.VANETs are special type of the wireless network in which communication through other intermediate vehicles(i.e.,vehicles with on-board units)on the road.The information can be delivered by exploiting the new connectivity opportunity provided by vehicular nodes which will store and forward data to the optimal next-hop forwarder until data is delivered to the destination.Since VANETs have unique and critical characteristics,such as: the vehicles may join or leave within one another transmission ranges abruptly or gradually.Due to the different vehicles travel on the roads with different directions and speeds,the network topology is highl y dynamic and consequently the communication links,the established wireless links between the vehicles may break and the communication link from the source to the destination vehicle is prone to disconnection.These unique characteristics of VANETs raise obstacles that should be resolved before deploying these networks.As a result of this mobility,VANETs have many challenging research issues,such as data dissemination,data sharing,and security issues.One of the most significant current discussions is the design of a reliable routing protocol to cope with complex and highly mobile environments in VANETs.The main requirement of VANET routing protocols is to achieve minimal communication time with minimum consumption of network resources.It requires simple and efficient forwarding mechanisms that can work with highly dynamic topology.Since,forwarding mechanisms are regularly selected an optimal next-hop forwarder and used by many routing algorithms to forward the data packets to the destinations.Ther efore,the performance of routing relies on the most suitable next-hop selection mechanism for data delivery among vehicles.It still not p erform well,if the routing can not find an appropriate next-hop vehicle,as which is essential entity for delivering packets from source to destination.However,existing geographic routing protocols often choose the next-hop forwarding vehicles according to traditional greedy forwarding by selecting the neighboring vehicle which is geometrically closer to the destination vehicle,regardless of the reliability of the newly formed link,which will give rise to great challenge in geographic routing to enter the local maximum problem.In addition,the information of vehicles is the critical factors to enhance the performance of the geographic routing protocols.To the best of our knowledge,direction and speed of vehicles have the most impact on the precision of the intelligent next-hop selection algorithms for the VANET routing protocols.The aim of this work is to derive th e information of vehicles in the Hello message and the data packet header within the optimal next-hop selection algorithms(ONSAs)over geographic routing protocols those are suitable for VANETs.The geographic routing protocol such as greedy perimeter sta teless routing(GPSR)uses the forwarding decision making by the greedy and perimeter forwarding schemes which are mainly depending on accurate position information of nodes.When position information is out-of-date,it may lead to wrong directions,and se lects the wrong next-hop forwarder node,thus the forwarding process may fail.Furthermore,the greedy forwarding scheme in the GPSR usually forwards the data packets to the next-hop node that is geometrically closer to the destination node.When the forwa rder is faced with an empty area,which means there are no neighbors closer to the destination than the forwarder itself,the greedy forwarding method will fail to forward the packets,in this case,called a local maximum problem.In order to mitigate the drawbacks of the GPSR and provide reliable and efficient packets forwarding mechanisms in various parameters of VANETs,the distinct conceptual frameworks are proposed.To achieve this global objective,the concept of the link reliability,cosine similarit y and speed similarity is adopted.The study of the movement direction and predictive distance of vehicles have been taken into account into consideration within the ONSA s.First,to escape from the local maximum situation,the combination of the greedy forwarding and link reliability model called an optimal next-hop selection algorithm based on the link reliability model(ONSA-LRM)is proposed.When the greedy forwarding scheme in the GPSR fails or a local maximum occurs,the source will choose the neighb or vehicle which has the highest value of longest lifetime of communication by estimation of link expiration time as a next-hop forwarder to forward the data packets from source to destination.Considering the various simulation parameters that fit to VANET environment,confirm that the proposed ONSA-LRM algorithm outperforms the existing geographic routing algorithms in terms of packet delivery ratio and average end-to-end delay.Besides,to avoid the wrong next-hop selection and reduce the forwarding of the data packet failure and improve the geographic routi ng performance,t he study of the movement direction and speed information of vehicles has been taken into account into consideration within an optimal next-hop selection algorithm based on the similarity model(ONSA-SM).The vehicle with largest similarity value is chosen as the suitable next-hop to forward a data packet.The simulation results show that the proposed ONSA-SM algorithm performs well in term of packet delivery ratio and avera ge end-to-end delay,compared to other existing next-hop selection algorithms.Then,we have proposed another optimal next-hop selection algorithm based on the movement direction model(ONSA-MDM)under GPSR.First,we select the candidate vehicle by considering the number of vehicles that move in the direction of the destination vehicle.We then consider the vector proje ction of each candidate into consideration to select the op timal next-hop node.Otherwise,instead of perimeter routing to recover the local maximum,we take into account the link-reliability between each neighbor and the current vehicular node into consideration based on the LET information.The neighboring vehicle with largest life time of co mmunication is chosen as the optimal next-hop to forward a data packet to the destination.The simulation results reveal that the proposed ONSA-MDM approach can achieve a better performance in terms of the packet delivery ratio and average end-to-end delay.Finally,we have proposed an optimal next-hop selection based on the predictive distance model(ONSA-PDM)that incorporates the predictive distance and link reliability models to take the forwarding decisions.To select an optimal next-hop,the neighbor that will be the closest to the destination in the near future is considered using the predictive distance model.In the local maximum situation,the vehicle's connectivity in its next-hop selection policy is also used,the neighboring v ehicle with largest value of link expiration time is chosen as the optimal next-hop to forward a data packet to the destination.In this framework,the proposed ONSA-PDM algorithm presents lower normalized routing load than other existing geographic routing algorithms,and t akes higher packet delivery ratio and throughput in varying VANET scenarios.Also,in the design of the network simulation it has a tremendous importance all the aspects that contribute to evaluate the performance of the VANET routing protocols.Therefore,we have worked in a bottom-up procedure step-by-step.First,we have studied the different components and tools that in actual communication scenarios with the vehicular mobility to support a higher degree of realism.In this work,the network topology fo r urban environment is considered.We have generated the VANET scenarios by VanetMobiSim-1.1,which aspects produce dramatic changes in the simulation results.Then,we have studied a network architecture,traffic generators,and propagation models to test and evaluate the performance of the new VANET routing protocols.We carried out the simulations in NS-2.35.Another important aspect to be considered when evaluating the VANET routing protocols is which performance metric should be used in order to repres ent an unbiased performance of t he routing protocols.Finally,w e have studied the impact of important factors such as the number and maximum speed of vehicles for comparing its performance with those of the existing geographic routing algorithms.The simulation results clearly show that the proposed optimal next-hop selection algorithms based on the link reliability,similarity metrics,movement direction and predictive distance model are viable and can significantly overcome the drawbacks of the existing geographic routing algorithms.The simulation results show that the proposed algorithms perform well in term of t he performance metrics values,compared to the other existing next-hop selection algorithms. |
PDF Full Text Request